Conventional ultrasound devices may be used for medical diagnosis, imaging, and stimulation. For example, ultrasound imaging generally involves transmission of high frequency sound into the body followed by the reception, processing, and parametric display of echoes returning from structures and tissues within the body. In addition, ultrasounds can be used to heat tissue, change cell membrane permeability, and enhance healing speed of a wound or bone fracture. Also, by measuring the echoes of ultrasounds bouncing back from a blood flow, the blood flow rate can be determined. By measuring the propagation speed of ultrasounds in tissue, the tissue temperature can be identified.
Unfortunately, traditional medical ultrasonic systems are bulky and normally operate outside the body. Some conventional intravascular ultrasounds (IVUS) can be inserted through blood vessels for examining vessels in the body and the arteries in the heart. However, its size (typically 3-4 millimeters in diameter) limits its use to larger vessels.
Image resolution from conventional ultrasound systems is relatively poor, especially for anatomical structures deep inside body tissue. Because of wavelength related penetration depth limit, long-wavelength ultrasonic waves are required for detection and/or stimulation of organs/tissues. For imaging or diagnosis applications, long-wavelength acoustic waves limit the image resolution or the accuracy of the measurement/diagnosis. For therapeutic stimulation, long-wavelength ultrasounds generally spread over an area larger than needed and could negatively affect the neighboring healthy tissue.
Short-wavelength waves are capable of providing images of higher resolution. They also are easier to focus on the target tissue in diagnosis or stimulation applications. However, limited by their short penetration depth, they cannot penetrate through the body to reach a target deep inside the tissue.
Conventional cryoprobes (for introduction of liquid nitrogen to freeze tissue), temperature-sensing probes, and/or an ultrasound imaging device may be provided for performing a cryosurgery. The temperature sensor and the ultrasound imager may be used to monitor the temperature and anatomical structure of tissue during a freezing and thawing process. These conventional probes and devices are typically of millimeter to centimeter diameter cross-section and cause significant disruption to the tissue. They are not suitable for operation in more delicate organs, for example, the brain and liver, because the disruption (and as a result the damage) is too much.
Conventional approaches for micromachining a silicon substrate to form miniature implantable devices include the dissolving wafer process using boron etch-stop and the dry-etching micromachining on a silicon-on-insulator (SOI) wafer. The dissolving wafer process is typically able to accurately control the silicon substrate dimension to within ±1 μm and has been used to fabricate electrode arrays and multi-channel drug delivery chips. The drawback of the boron-defined dissolving wafer process comes from the diffusion process required for doping the silicon and the high tensile stress in the boron-doped silicon. The high-temperature boron diffusion process may not be very compatible with transistor and/or MEMS processes. Additionally, the boron-caused high-stress may warp the device structure. The SOI micromachining may utilize double-sided dry etching to form a probe-shaped silicon substrate on the silicon layer of a SOI structure. The device thickness is determined by the thickness of the SOI structure. The thick substrate under the buried oxide is removed by a backside deep silicon etching. While this approach may provide good control on the final device dimension and prevents the stress problem, it requires use of a high-cost SOI wafer.
The present disclosure describes exemplary embodiments of medical devices, fabrication methods, and medical procedures that may solve one or more of the above problems.